My materials room (Poser 12 on a Mac) suddenly re-arranged itself, with the view window and the nodes window squeezed up high on the screen, with the two of them covering the whole width of the screen, like this:

Even if I quit Poser, and start it up again with a totally new scene, the Material Room is still arranged like this.

How do I get it back to the way it used to be?

On the Top Menu go to Poser > General Preferences ...

ON the Interface tab put the dot on Launch to factory state.

[OK]

Exit and restart poser.

That gets you back to the factory UI. You can safely return the preference to "Launch to previous state" in preferences.

Now how to make sure this never happens again ...

Every room in poser has a set of memory dots. Among other things they store the UI layout for that room.

A Dark dot is empty.

A light dot contains a stored layout

A blue dot is the layout that's applied.

Click a dark dot to store a layout

Click a light dot to load a layout.

[ALT] + Click a dot to erase what's stored in it.

Once you've reset your UI you should store the factory UI in one of the dots.If it ever gets messed up a gain your one click from the factory layout. Then arrange your pallets to your liking and store that in another Memory Dot. Next thing you know you'll be storing different UI arrangements for different tasks.

Every thing about Memory Dots: https://www.posersoftware.com/documentation/12/index.htm#t=Poser_Reference_Manual%2FBuildingScenes%2FPoseRoom%2FMemory_Dots.htm&rhsearch=memory%20dots&rhhlterm=memory%20dots%20dot

That did it!

Many thanks!